1. The first two characters should be your status. DM = Diamond,PM = Platinum, GM = Gold, FO= Silver, SY = General Member, and blank is considered SY.
2. The next two characters are RT for Round Trip. This will take your current list of cities and calculate your travel back to your origin airport.
3. IATA Airport Code.  The three letter airport code.
4. Fare Class.  Your fare class of service.
5. If you wish to compare two itineraries use a comma ex. ATL SFO, ATL SJC
6. If you are flying out of a different airport use "XX"  Ex. ATL SJC XX SFO ATL 
7. Repeat steps 3 and 4 to your heart's content :-)

Examples:

1. ATL JFK (Atlanta to JFK)
2. ATL SFO, ATL SJC (Compare miles for Atlanta to San Francisco AND Atlanta to San Jose
3. ATL SFO XX SJC ATL (Atlanta to San Francisco and returning San Jose to Atlanta
4. DM RT DAB BWI (A Diamond Medallion on a round trip ticket from Daytona Beach to Baltimore
5. PM ATL SFO HNL LAX ATL (A Platinum Medallion going to Atlanta, San Francisco, Honolulu, Los Angeles
6. GM SFO Y ATL K LGW (A Gold Medallion traveling from San Franciso to Atlanta on a "Y" fare and Atlanta to London Gatwick on a "K" fare
7. FO RT ATL B SEA (A Silver Medallion traveling from Atlanta to Seattle on a "B" Fare